Digital watermarking is the science of encoding physical and electronic objects with plural-bit digital data, in such a manner that the data is essentially hidden from human perception, yet can be recovered by computer analysis. In physical objects, the data may be encoded in the form of surface texturing, or printing. Such marking can be detected from optical scan data, e.g., from a scanner or web cam. In electronic objects (e.g., digital audio or imagery—including video), the data may be encoded as slight variations in sample values. Or, if the object is represented in a so-called orthogonal domain (also termed “non-perceptual,” e.g., MPEG, DCT, wavelet, etc.), the data may be encoded as slight variations in quantization values or levels. The present assignee's U.S. Pat. Nos. 6,122,403 and 6,614,914 are illustrative of certain watermarking technologies.
Watermarking can be used to tag objects with a persistent digital identifier, and as such finds myriad uses. Some are in the realm of device control—e.g., tagging video data with a do-not-copy flag that is respected by compliant video recorders. (The music industry's Secure Digital Music Initiative (SDMI), and the motion picture industry's Copy Protection Technical Working Group (CPTWG), are working to establish standards relating to watermark usage for device control.) Other watermark applications are in the field of copyright communication, e.g., indicating that an audio track is the property of a particular copyright holder.
Other watermark applications encode data that serves to associate an object with a store of related data. For example, an image watermark may contain an index value that serves to identify a database record specifying (a) the owner's name; (b) contact information; (c) license terms and conditions, (d) copyright date, (e) whether adult content is depicted, etc., etc. (The present assignee's MarcCentre service provides such functionality.) Related are so-called “connected content” applications, in which a watermark in one content object (e.g., a printed magazine article) serves to link to a related content object (e.g., a web page devoted to the same topic). The watermark can literally encode an electronic address of the related content object, but more typically encodes an index value that identifies a database record containing that address information. U.S. Pat. No. 6,947,571 details a number of connected-content applications and techniques.
One problem that arises in many watermarking applications is that of object corruption. If the object is reproduced, or distorted, in some manner such that the content presented for watermark decoding is not identical to the object as originally watermarked, then the decoding process may be unable to recognize and decode the watermark. To deal with such problems, the watermark can convey a reference signal. The reference signal is of such a character as to permit its detection even in the presence of relatively severe distortion. Once found, the attributes of the distorted reference signal can be used to quantify the content's distortion. Watermark decoding can then proceed—informed by information about the particular distortion present.
The assignee's U.S. Pat. Nos. 6,614,914 and 6,408,082 detail certain reference signals, and processing methods, that permit such watermark decoding even in the presence of distortion. In some image watermarking embodiments, the reference signal comprises a constellation of quasi-impulse functions in the Fourier magnitude domain, each with pseudorandom phase. To detect and quantify the distortion, the watermark decoder converts the watermarked image to the Fourier magnitude domain and then performs a log polar resampling of the Fourier magnitude image. A generalized matched filter correlates the known orientation signal with the re-sampled watermarked signal to find the rotation and scale parameters providing the highest correlation. The watermark decoder performs additional correlation operations between the phase information of the known orientation signal and the watermarked signal to determine translation parameters, which identify the origin of the watermark message signal. Having determined the rotation, scale and translation of the watermark signal, the reader then adjusts the image data to compensate for this distortion, and extracts the watermark message signal as described above.
With the foregoing by way of background, the specification next turns to the various improvements. It will be recognized that these improvements can typically be employed in many applications, and in various combinations with the subject matter of the patent documents cited herein.